In many systems, it is desirable to measure relative displacement between two members. One example is a probe type data storage device. Probe type data storage devices can provide high storage capacity in a very small form factor (e.g., an SD card). Such devices include an array of probe tips, or heads, that are used to read and write data to a storage media, such as a ferroelectric media. Each tip may operate on a specified area of the storage media. Relative movement between the media and the tips can be implemented using actuators, such as electromagnetic actuators or electrostatic actuators, which are coupled to the media or a support structure for the media or the tips. In one example, the media can be moved in first and second directions (i.e., scanning and tracking directions) that are orthogonal to each other. Data are stored in tracks on the storage media. In the scanning direction the actuators move the media at a resonant frequency to achieve a high data rate, while in a tracking direction a tracking control is used to enable data storage in a high number of tracks per inch.
The desired track width can be on the order of tens of nanometers, which places difficult requirements on the accuracy of a servo system, which controls the relative position of the media and the tips. In order to achieve this nano-precision servo accuracy, a sub-nanometer precision position sensing is needed. Position sensing can be accomplished using a combination of capacitive sensors and embedded servo marks. The capacitive sensors may provide the primary sensing of a relative position of the tips and media, and the embedded servo marks on a non-active part of media may provide secondary sensing of the position of each tip.